1

Evolution IPopulation Genetics

Evolution is a genetic process. A change in the geneticstructure of a single population, or the formation of new geneticlineages.

2

Evolution has two components:

Anagenesis - change in a genetic lineage through time - A smallor large change can be considered evolution. A slight increase ordecrease in the frequency of a particular blood type for examplecould be considered evolution.

Cladogenesis - the origin of new genetic lineages (new species).Although evolution occurs within lineages, two populations of asingle species can become so different that they no longer belongto the same genetic lineage.

Anagenesis is the more commonly appreciated component ofevolution.

Since evolution is a property of populations and it is a geneticphenomenon, understanding evolution requires an understandingof how genes behave in populations.

3

The fundamental principle of population genetics is the Hardy-Weinberg Principle:If the individuals in a large population mate randomly, andthere is no mutational change in the frequency of alleles in thepopulation, and there is no influx of genes from otherpopulations, and there is no difference in the survival andreproductive potential of different genotypes, then thefrequency of genotypes in the population will be a binomialdistribution derived from the frequency of the two alleles.If the frequency of A = p = 0.5, and the frequency of a = q = 0.5,then the frequency of the genotypes will be:f(AA) = p2 = 0.5*0.5 = 0.25f(Aa) = 2pq = 2*0.5*0.5 = 0.5f(aa) = q2 = 0.5*0.5 = 0.25

And, so long as the conditionsstated above hold, thefrequencies will not changefrom generation to generation.

4

Finding the frequency of an allele in a population, and findingthe HW genotypic frequencies.If there are two alleles B and b, then there are three genotypesBB, Bb, and bb.If f(BB) = D, if f(Bb) = H, and f(bb) = R thenf(B) = D + 1/2 H = p, and f(b) = R + 1/2 H = qHW frequencies are f(BB) = p2, f(Bb) = 2pq, f(aa) = q2

5

Example: Frequency of cystic fibrosis

The allele that causes cystic fibrosis is recessive, and its frequencyin Caucasian Americans is 0.022. Assuming there is only oneallele that is dominant to the cystic fibrosis allele, then:

f(c) = q = 0.022 f(C) = p = 1-q = 0.978

What is the expected frequency of the different genotypes amongCaucasian Americans?

f(CC) = 0.978 * 0.978 = 0.956

f(Cc) = 2 * 0.978 * 0.022 = 0.043

f(cc) = 0.022 * 0.022 = .0005

About 5 in 10,000 births among Caucasian Americans shouldhave cystic fibrosis, and about 4.3% of the same group should becarriers of cystic fibrosis.

6

Evolution is the product of factors, conditions, and forces thatchange allele frequencies

Mutation - change of one allele to another

A → a

Mutation can change allele frequencies, butthe rate of mutation is normally so low (onaverage about 1 mutation in a given gene inevery 1,000,000 gametes) that the effect ofmutation is not measurable and it usuallyhas a negligible effect on the frequency ofalleles.

Mutation does create new alleles that canbecome common as a result of other causesof evolution.

7

Immigration - the influx of alleles from other populations - alsocalled gene flow.

If members of one population regularly enter another populationthe frequency of alleles in the receiving population may change tobecome more like the source population.Gene flow tends to homogenize populations- to make them more similar genetically.Only small numbers of individuals need tobe exchanged for the two populations tobecome genetically indistinguishable in ashort period of time.

Gene flow can be a powerful force forevolutionary change, and gene flow cansometimes oppose other forces ofevolutionary change.

8

Nonrandom mating - also known as assortative mating -results in deviations from the Hardy-Weinberg expected ratios.

Inbreeding (mating among close relatives), or self-fertilization, orlike mating with like all lead to a reduction in the frequency ofheterozygotes in a population, and an increase in the frequency ofhomozygotes.

Inbreeding results in a higherfrequency of homozygotes fordeleterious alleles and can interactwith selection to reduce thefrequency of deleterious alleles in apopulation.

9

Small population size can lead to genetic drift - chance changein frequency of alleles in a population.

If a population remains small, then in each generation chancefactors may result in some genotypes not mating in theproportions expected. As a result, some genotypes may increasein frequency quickly while other decrease in frequency.

Even if a population is large, if few individuals produce the nextgeneration, the alleles those individuals have will be more highlyrepresented.

One type of event that causes genetic drift is thefounding of new populations by a fewindividuals. If the founding individuals aresomehow genetically different from the parentpopulation then the result is called a foundereffect.

10

Genetic drift, due to persistent small population sizes, foundereffects, or bottleneck effects, can result in large change in thegenetic composition of a population and commonly results in theloss of alleles from a population. It can also result in populationsthat have lost variation that may be important for long-termsurvival.

Genetic drift can be a potent force for evolutionary change.

Genetic drift can also occurwhen large populations arereduced to small sizes. Thechance change in geneticcomposition of a populationas a result of a greatreduction in size is called thebottleneck effect.

11

Selection - differences in survival and reproductive potential thatresult in differences in reproductive success. If some individualsin a population produce more offspring than others, eitherbecause they are better able to survive to adulthood, or are moreefficient at producing offspring, their offspring will berepresented in greater frequency in the next generation than othermembers of the population.

Selection ≠ EvolutionSelection (like genetic drift, or mutation) is a process. Evolutionis a potential outcome of several different processes.For selection to produce evolution there must be variation in thepopulation, the variation results in some individuals havingbetter survival or reproductive abilities than others, and thevariation must be heritable (due to underlying geneticdifferences).

12

The evolution of long necks in giraffes (a hypothesis).Ancestors of the giraffe had shorter necks than modern giraffesand browsed on tree leaves and shrubs of the African Savannah.Some had longer necks than others and were ableto reach higher branches than others. In times offood shortage, those with the longest necks had asurvival advantage. If so, selection has occurred.If neck length variation was not geneticallybased, then the next generation would not havenecks any longer than the average in the previousgeneration. No evolution would have occurred.If neck length was due to genetic variation, then the genes forhaving a long neck would have increased in the population. Thenew generation would be different from the prior generation, andbetter suited to survive in their environment - due to their necklength. This is an adaptation.

13

The power of selection to produce change is clear in thedevelopment of different breeds or varieties of domestic plantsand animals. For example, all of the breeds of domestic dog havebeen produced from a single ancestral genetic population throughthe action of humans preserving some characteristics andeliminating others during the development of a breed.

When humans are the source of selectionwe call it artificial selection. Whennatural events result in differences insurvival or reproductive success we call itnatural selection.

Predators can choose different phenotypespreferentially, because they stand out orbecause they are in some way moreattractive.

14

Crypsis (camouflage coloration) inprey species is the product ofnatural selection by predators

15

Many different aspects of theenvironment select for or againstsome types of phenotypicvariation and the genes that resultin the different phenotypes.Cold-adapted and warm-adaptedorganisms are selected atdifferent latitudes

The application of pesticides inagriculture produces pests that areresistant to specific pesticides.

Similar selection occurs in humanpathogens through the application ofantibiotics.

16

Differences in survival and reproductive ability are commonlycalled differences in fitness. So, natural selection is the result ofdifferences in fitness that are associated with differentphenotypes. Evolution by natural selection is the result of fitnessdifferences that are associated with phenotypic differences thathave a genetic basis.

Natural selection is often defined as “survival of the fittest.”

There are problems with this definition:1. Even if every individual was genetically and phenotypicallyidentical (no fitness differences), some would, by chance, surviveand reproduce while others would not. In such cases there is noselection even though some survive and others do not.2. It is a tautology - a truism - a statement that is always true.Who survives? Who are the fittest? The answers are circular.Natural selection is best defined as the process of differences insurvival or reproductive ability among different phenotypes.

17

Example: Selection among different genotypes in humans

Sickle cell anemia is due to a single base pair change in β-Hb.Without the aid of modern medicine, the sickle cell homozygotedies very young. So, why isn’t the sickle cell allele removed fromthe human population?

The sickle cell alleleand sickle cell anemiaare both mostcommon in areas ofthe world wheremalaria is or wasformerly common.

18

Malaria is caused by a protozoan blood parasite (Plasmodiumfalciparum) that is spread by Anophales mosquitoes.

The three human genotypes have different fitness where malariais common

The non-sickle cell homozygote (SS) is susceptible to malaria,about 20% of the SS individuals die of malaria where malaria iscommon.

The sickle cell homozygote (ss) is resistant to malaria but 100%die to the complications from sickle cell anemia

The sickle cell heterozygote (Ss) is resistant to malaria and doesnot have the problems associated with sickle cell anemia

The sickle cell allele is maintained at relatively high frequenciesin areas where malaria is common because of heterozygoteadvantage.

19

Types of selectionSelection can be classified based on its effects on a distribution ofphenotypes.

Directionalselection - occurswhen fitness ishighest amongindividuals withextreme phenotypes.If there is a geneticbasis for the fitnessdifferences, the resultwill be a shift in thedistribution ofphenotypes.

20

Stabilizing selection - selection against the extremes, or selectionfavoring the middle, of a distribution. Stabilizing selection tendsto reduce the frequency of extreme phenotypes.

21

Example: Skin color selection in humansWhy does human skin color vary among indigenous peoples overthe surface of the globe? Darker in equatorial zones and lightertoward the poles.Human skin color is controlled by 5 to 8 genes, each with smalleffects on the phenotype. They add to, or subtract from, theproduction of melanin.The amount of melanin in the skin influences several aspects ofhealth.Melanin is natural sunscreen - dark skinned humans are moreresistant to the damaging effects of ultraviolet radiation. UVlight causes mutations and skin cancer. UV light also destroysthe vitamin, folic acid. Low folic acid concentrations in mothersresults in birth defects.UV light is necessary for vitamin D production. Low vitamin Dproduction results in skeletal abnormalities.

22

Light skinned peoples are more susceptible to the sunlightinduced cancers and to folic acid deficienciesDark skinned peoples are more susceptible to vitamin Ddeficiencies.In the tropics - dark skinned people have higher fitness due toresistance to the damaging effects of sunlight (directionalselection)Toward the poles - lighter skinned people have higher fitness dueto sufficient vitamin D production. (directional selection)In temperate zones - intermediate skinned people are favored, dueto the problems of sunlight damage on the very light skinned, andvitamin D deficiencies in the very dark skinned. (stabilizingselection)This example illustrates that fitness depends on the environmentalcontext, and that high fitness is often the result of a compromise.

23

Disruptive selection - selection that favors the extremes of adistribution.

Disruptive selection the Apple Maggot Fly.

24

Sexual Selection - selection due to differential ability to obtainmates - because of preference by members of one sex for certainphenotypes in the other, or because of competition amongmembers of one sex for access to members of the other sex.

Competition usually occurs among malesof a species. Selection favors traits that aidthe male in his ability to battle rivals -horns, antlers, large canines, large bodysize.

25

Sexual selection by mate choice usually occurs due to femalechoice of males with certain characteristics

Females choose distinctivetraits in males like brightcolors and loud calls.

26

Why should females be the choosier sex?

Why choose bright colors, loud calls, etc?